Add RequiredDeep type

[jarpoole]

Closes #273

This PR is based on a past attempt (see #401) with all the feedback integrated. The only exception is that I did not change args to arguments because as was helpfully noted this is an error inside a strict context like a module.

[jarpoole]

CI fails because of xojs/xo#718. Seems like this was fixed in sindresorhus/eslint-plugin-unicorn#2076. I don't really understand what is going wrong here...

[sindresorhus]

Is this case handled? #401 (comment)

[sindresorhus]

Should it handle properties that are promises? I looked at some different implementations of this kind of type and noticed this one handles promises: https://github.com/ts-essentials/ts-essentials/blob/master/lib/deep-required/index.ts

[jarpoole]

Should it handle properties that are promises? I looked at some different implementations of this kind of type and noticed this one handles promises: https://github.com/ts-essentials/ts-essentials/blob/master/lib/deep-required/index.ts

Yes! I have added support.

[jarpoole]

Is this case handled? #401 (comment)

I think I've handled that case now but I'm not 100% sure.

[sindresorhus]

Use the type in source/internal.d.ts.

[sindresorhus]

Thanks :)